The human skeleton is composed of 206 individual bones that perform a variety of important functions, including support, movement, protection, storage of minerals, and formation of blood cells. To ensure that the skeleton retains its ability to perform these functions, and to reduce pain and disfigurement, bones that become fractured should be repaired promptly and properly. Typically, a fractured bone is treated using a fixation device, which reinforces the fractured bone and keeps it aligned during healing. Fixation devices may take a variety of forms, including casts and fixators for external fixation, and bone plates and bone screws for internal fixation, among others.
Bone plates are implants that may be positioned under skin and other soft tissue for mounting on the bone adjacent the fracture. These plates may be manufactured and/or custom bent for mounting to particular regions of bone. To use a bone plate to repair a fractured bone, a surgeon (1) selects an appropriate plate, (2) reduces (sets) the fracture, and (3) fastens the plate to the bone on opposite sides of the fracture using suitable fasteners, such as bone screws, so that fragments of the bone are fixed in position.
Bone plates generally include a plurality of apertures sized and shaped to receive fasteners, such as bone screws. Each aperture of a bone plate may function as a nonlocking aperture or a locking aperture.
A nonlocking aperture generally receives a nonlocking fastener without threaded (or other locking) engagement between the bone plate and the fastener. For example, the nonlocking aperture may receive an undersized “toggle” fastener at a range of permitted angles or a close-fitting fastener at a predefined, fixed angle. In either case, the nonlocking fastener relies on bone to maintain engagement between the nonlocking fastener and the bone plate, for example, with the nonlocking fastener holding the bone plate in position against bone by engagement of the underside of a head of the nonlocking fastener with the bone plate.
A locking aperture may receive a locking fastener such that the locking fastener is disposed in a locked configuration. The locked configuration may be provided by a thread-engagement structure (such as an internal thread) of the locking aperture that is meshed with an external thread of the locking fastener. The locking fastener thus may be restrained from translational motion relative to the bone plate in both axial directions defined by the long axis of the locking fastener, to hold the bone plate in position and restrict plate motion both toward and away from bone.
Locked engagement and nonlocked engagement of fasteners with bone plates both have advantages for particular applications. However, each aperture of a bone plate generally is configured and/or optimized for use with either a nonlocking (e.g., toggle) fastener or a locking fastener, but not both.